1. Field of the Invention
The present invention relates to a head gimbal assembly on which a magnetic head slider having a microwave-assisted magnetic recording system is mounted.
2. Description of the Related Inventions
There is a demand for improvement in recording density of magnetic disk devices that are magnetic recording devices. In order to ensure the required signal quality (signal to noise (S/N) ratio) in high density recording, there is a need to reduce the size of magnetic particles that configure a magnetic recording medium in conjunction with the improvement of surface recording density. However, the magnetic particles having reduced size are more likely to cause a magnetization disappearance due to heat fluctuation. In order to prevent this problem and maintain a stable recording state, there is a need to increase magnetic anisotropy energy of the magnetic particles. When a material with high magnetic anisotropy energy is used, coercive force of the magnetic recording medium is increased, and therefore, a strong recording magnetic field becomes necessary to record to the magnetic recording medium. Meanwhile, the intensity of magnetic fields generated by a recording head element is limited by the material and shape of the recording head element, which makes recording with an intense recording magnetic field difficult.
In order to resolve this technical problem, energy assisted recording has been proposed in which, at the time of recording, supplemental energy is applied to a magnetic recording medium to lower effective coercive force. A recording system using a microwave magnetic field as a supplemental energy source is called microwave assisted magnetic recording (MAMR). Jian-Gang Zhu, Xiaochun Zhu, and Yuhui Tang, ‘Microwave Assisted Magnetic Recording’ IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 1, JANUARY 2008 should be referred.
In microwave assisted magnetic recording, the system of supplying a microwave magnetic field by a microwave oscillator arranged in a tip end of a magnetic head, and the system of supplying a microwave excitation current, the signals being supplied from a microwave signal generating circuit that is independent from the magnetic head, to a microwave radiation element are known. The latter is called external oscillation system microwave assisted magnetic recording. With this system, because a microwave excitation current is supplied to a microwave radiation element that is formed near a recording head element of a magnetic head slider, a microwave transmission line should be provided on a suspension. Here, the suspension indicates a portion of a head gimbal assembly from which the magnetic head slider is excluded, which is, in other words, a support structure for the magnetic head slider.
In order to efficiently transmit a microwave excitation current, a high frequency transmission loss should be suppressed. U.S. Patent Application Publication No. 2011/0090602 discloses a microwave transmission line for efficiently supplying a microwave excitation current to a microwave radiation element of a magnetic head slider. Shields are provided above and below the microwave transmission line, and a conductive column that connects the shields is provided. By adjusting the position of the conductive column, the characteristic impedance of the microwave transmission line is controlled, and a microwave excitation current is accurately and efficiently transmitted.
JP Laid-Open Patent Application No. H09-251813 discloses a method in which a signal line of a flexible printing cable (FPC) is configured with a plurality of conductive lines that are connected in parallel so as to decrease the inductance of the signal line so that the high frequency transmission loss is suppressed.
The technologies described in the above-described documentations allow the control of the characteristic impedance of the microwave transmission line. However, in order to efficiently supply a microwave excitation current to the microwave radiation element, only the control of the characteristic impedance of the microwave transmission line on the suspension is insufficient. Specifically, it is required to suppress a reflection loss due to impedance mismatching between the microwave transmission line and the microwave radiation element.
Because the microwave radiation element is normally formed by a minute conductive line, an electrical resistance component thereof is extremely small. Accordingly, it is difficult to achieve impedance matching between the input impedance seen looking into the microwave radiation element at a terminal electrode of the microwave radiation element provided on the magnetic head slider and the input impedance seen looking into the microwave transmission line on the suspension at the terminal electrode.